The present invention relates to a copier, printer, facsimile machine or similar electrophotographic image forming apparatus and, more particularly, to an image forming apparatus having a mechanism for collecting a toner left on a photoconductive element after image transfer.
An image forming apparatus of the type described usually includes a photoconductive element implemented as a drum. Arranged around the drum are a device for electrostatically forming a latent image on the drum, a device for developing the latent image, a device for transferring the developed image, or toner image, from the drum to a paper, a device for cleaning the drum, i.e., removing a toner remaining on the drum, a device for conveying the toner removed by the cleaning device to the developing device, a device for feeding a paper to the transferring device via a registration roller, and a fixing device. The paper reached the registration roller is driven toward the drum at a predetermined timing in synchronism with the rotation of the drum. As the leading edge of the paper abuts against the drum, the drum causes it to electrostatically adhere thereto and transports it. At this instant, the paper closely adheres to the drum due to the elasticity thereof and an electrostatic force. In this condition, the transferring device transfers a toner image from the drum to the paper.
An image forming apparatus of the type using a recording body implemented by an endless dielectric film or belt is disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 179879/1982 by way of example. This type of apparatus locates recording electrodes between the opposite runs of the belt and disposes a developing device using a magnetic toner outside of the belt. A voltage is selectively applied to the recording electrodes to form a toner image on the belt. The toner image is transferred from the belt to a paper. After the image transfer, the charge of the toner left on the belt is dissipated. Subsequently, the toner deposited on the belt is magnetically removed by and collected in the developing device.
The problem with an image forming apparatus of any of the above-described types is impurities which include paper dust produced when a paper is cut, paper fibers produced when a paper is perforated or punched, and talc, resin and other components of a paper. Such impurities are produced in a great amount when, among others, a great number of papers are used or when papers of the kind producing a great amount of paper dust are used. The impurities electrostatically adhere to the surface of the photoconductive element in the event of image transfer. Further, when the trailing edge of a paper shakes at the end of image transfer, paper dust produced from the cut edge of the paper deposits on the background of the photoconductive element. In this condition, when the cleaning device removes the toner remaining on the photoconductive element, the paper dust and other impurities stop the gap between the element and the edge of a cleaning blade, thereby degrading the cleaning ability. Moreover, when the collected toner is returned to the developing device, the impurities are apt to deposit, grow and then collapse on the inner periphery of the casing of the developing device. This part of the toner is caught by a doctor included in the developing device, resulting in defective images. Further, the collected toner contains, in addition to the impurities, a toner of relatively small particle size, a toner pulverized during the course of development or image transfer, and a toner whose chargeability is different from the chargeability of the toner to be supplied. When such a composite toner is returned to the developing device, the pulverized toner coheres on the impurities, or cores, and lowers the image quality.
In the light of the above, a mesh may be disposed in a path along which the collected toner is transported to the developing device, as proposed in the past. This kind of scheme, however, needs vibrating means, a replacing mechanism, cleaning means, a slide mechanism and so forth to prevent the mesh from being stopped up, complicating the overall structure of the apparatus. In addition, the mesh scheme lowers the reliability of the apparatus by bringing about toner scattering, toner blocking and other undesirable occurrences. Alternatively, use may be made of an electric field or a pneumatic pressure. Further, a device capable of removing paper dust from a paper before image transfer may be used, as disclosed in Japanese Patent Laid-Open Publication No. 55128/1978. However, even such alternative schemes complicate the overall structure of the apparatus, increase the size and cost of the apparatus, and need a broad area for installation.